In 1984 (the most recent year data is available), power production at U.S. utility plants resulted in about 80 million tons of solid waste of which an estimated 72-76 million tons was produced from coal and oil combustion. Of the latter, about 53 million tons was fly ash from coal combustion, 19 million tons was bottom ash, and 5 million tons was sludge from flue gas scrubbers.
About 75% of this utility solid waste is disposed in landfills or surface impoundments, with only 25% being used as construction or road building material. Utilities operate over 1,000 waste disposal facilities, at a cost of around $800 million per year. By the year 2000, the total of fly ash, bottom ash and FGD sludge wastes is expected to increase to about 125-150 million TPY. Further, because of the expected more stringent regulations for landfills and more extensive groundwater monitoring, the disposal costs are expected to double.
Examples of typical coal ash constituents are found in the Coal Ash Disposal Manual, 2d Ed., Final Report October 1981, CS2049 Research Project 1685-3 by GAI Consultants Inc. for the Electric Power Research Institute, Palo Alto, Calif., and in U.S. Pat. No. 3,962,080. These references report that inorganic compounds of Si, Al, Fe, Ti, Ca and Mg comprise over 90% of coal ash in the form of unburned mineral matter (rock). But the focus is on toxic trace amounts of heavy metals (such as B, Ba, Be, Pb, Co, Cu, Cd, Cr, Zn, Zr, Se, Sn, and As), naturally occurring radionuclides (principally Ra.sup.226, Ra.sup.228 /Th.sup.228, Th.sup.230 and Th.sup.232), and organics such as polycyclic aromatic hydrocarbons (PAH's) that are part of the residual (unburned) fuel carbon. The fly ash and bottom ash are essentially indistinguishable in these toxic components.
The present methods of disposal of such ash components in open ponds or landfills permit slurry water, rain or ground water to leach those toxic elements from them and allows the elements to migrate into the ground water. This is particularly the case where the leaching water contains natural, or ordinarily present, chemicals (e.g. acid rain, or acid or caustic sludges) which tend to accelerate leaching. In view of the hazards involved with such toxic trace elements or PAH's entering the groundwater, the utility industry is researching the leachability of ash and sludge wastes. That industry is also vitally interested in methods of inhibiting leach water migration, such as by the use of pond and landfill bottom and top liners.
Still another concern in the utility industry involves desulfurization of flue gas using reactive sorbents to remove sulfur oxide compounds from the flue gases to meet EPA emission standards. For example, 60-70% of the sulfur in low sulfur Montana coal (containing about 0.5% total S) must be removed to achieve compliance. There is also impetus for better FGD processes. This impetus is derived from the growing concern over acid rain problems traceable to stack gas SO.sub.x emissions. Currently, the principal FGD process involves wet limestone scrubbing which results in approximately 5 million TPY of disposable calcium sulfate scrubber sludge. Less than 1% of this sludge is utilized in any meaningful fashion (typically for gypsum-type wallboard). This wet sludge is usually disposed of in ponds, but for dam safety reasons the disposal ponds are rarely more than 20 feet (6 meters) deep. In contrast, dry landfill materials can be compacted to a denser product and can be stacked over 100 feet (30 meters) in height. Because of poor structural properties, the calcium sulfate sludge must be dewatered and stabilized before it is deposited in a landfill.
Further, there is increased interest in the use of sodium compounds instead of calcium as the FGD sorbent because of: (1) the higher reactivity of sodium compounds; (2) the lower capital investment; and (3) the greater ease of retrofiting sodium FGD equipment to existing plants. This is particularly true in the case of dry FGD processes involving the use of Nahcolite, a naturally occurring sodium bicarbonate mineral, which is introduced in a dry powdered form into the flue gas of a power plant. The Nahcolite reacts with the SO.sub.x in the flue duct and is collected in a baghouse or electrostatic precipitator. Nahcolite can remove from 70% to well over 90% of the SO.sub.x at stoichiometry, depending upon the flue gas temperature. The Nahcolite process has the advantage of producing a dry waste product. In contrast, wet scrubber sludges, being difficult and expensive to dewater, must be disposed of in ponds, or if landfill disposal is desired, must be chemically processed to provide the stability required for the landfill because of their poor structural integrity.
There is some concern over the relative solubility of sodium FGD wastes, which are predominately sodium sulfate. One solution is shown in U.S. Pat. No. 3,962,080, which is directed to the SINTERNA process involving sintering a conglomerate of FGD waste plus fly ash. The resultant sintered pellets have lower solubility than calcium sulfate scrubber sludges; they also have a variety of commercial uses or may be disposed in simple landfills. Another solution to sodium solubility is shown in U.S. Pat. No. 3,984,312 directed to the FERSONA process in which the sodium FGD waste is reacted in solution with ferric ions and sulfuric acid to produce an insoluble basic hydrous or anhydrous sodium hydroxy ferric sulfate or sulfite compound. These compounds are insoluble compared to scrubber sludge calcium sulfate. Both these processes involve additional thermal or chemical processing prior to disposal.
Another approach to the disposal of the sodium FGD waste is isolation clay landfill. In The Use of Nahcolite Ore and Bag Filters For Sulfur Dioxide Emission Control, ACPA Journal, Vol. 25, No. 12, December 1975, pp 1244-1253, at 1252, there is shown a process for disposal of sodium FGD waste in special landfills. An impermeable base of material is required over which is emplaced a day's production of FGD waste from the power plant. That waste is then covered with a layer of clay on the order of 1 foot thick, forming an isolated clay cell. Each day's production is thus sealed in a clay envelope, generally romboidal in cross section. Each succeeding day's production rests in part on the cover and side layers of the preceding day's production. The final cover for the top is a 3 foot thickness of clay-like material, and the sides require a 4 foot layer. While this approach may be environmentally approvable in certain situations, there is concern over the potential for breaching the integrity of the overlying or underlying clay envelope walls, followed by subsequent leaching of the soluble sodium sulfate therefrom by rain water. In addition, there is some fugitive dust problem during the emplacement of the very dry FGD waste. There is also concern over the potential for cracking-open of the envelope(s) if a portion becomes wet and the sodium sulfate expands as it hydrates. This expansion could cause heaving and cracking, thus breaching the integrity of the landfill.
Reginato, et al. in Sodium Carbonate for Reducing Seepage from Ponds, U.S. Water Conservation Laboratory, WCL Report 7, Jan. 1, 1968, 7 pages, recommend treating stock ponds having large quantities of calcium in the water and soil with tech grade granular soda ash to reduce seepage. The soda ash is worked into the bottom and sidewalls of the dry point to a depth of 4 inches by disking or harrowing. After receiving water, soda ash is added to the water in an amount not exceeding 0.88 lbs/1000 gallons of water (so cattle can still drink the water). Additional sodium is supplied by adding NaCl to the water. This procedure reduces the pond seepage from 5 inches/day to 0.1 inch/day, a 50-fold reduction in seepage.
While produced in relatively small quantities today, the amount of FGD waste generated will be increasing rapidly as FGD processes become adopted by industry and utilities. Thus, the utility waste problem will be increasing, not only by the changes in regulation requirements and enforcement, but also by the addition of FGD wastes to be disposed in addition to the fly ash and bottom ash.
Accordingly there is a great need in the industry to provide an improved process for disposing of fly ash alone or in combination with a dry FGD waste, particularly the sodium FGD wastes.